1. Field of the Invention
The present invention relates to a tracking control apparatus for an optical disk device which records and reads data to/from a recording carrier having plural data tracks using an optical beam emitted from a semiconductor laser diode or other source.
2. Description of the Prior Art
FIG. 1 is a block diagram of a tracking control apparatus in a conventional optical disk device. The recording carrier 1, such as a disk, is mounted on the spindle of motor 2, and thereby driven at the rated speed. The signal is recorded to a spiral track imprinted to the recording carrier 1. The track width in this example is 0.6 .mu.m wide, and the track pitch is 1.6 .mu.m.
The optical beam 4 emitted from the light source 3, which is often a semiconductor laser diode, is modified to a parallel beam by the coupling lens 5, passed through the polarization beam splitter 6 and 1/4 wavelength plate 7, and reflected by the total reflection mirror 8 which is mounted on a carrier 13. The reflected beam is then focused by the objective lens 10 on the recording carrier 1.
The objective lens 10 is mounted on a movable member 40 and the optical axis of the objective lens 10 is parallel to the shaft 51 to which the movable member 40 is rotatably and slidably mounted. The movable member 40 comprises bearings enabling it to move vertically along sliding motion shaft 51, which is mounted on a carrier 13, and can pivot from a neutral position, around the sliding motion shaft 51. The movable member 40 is mounted to the carrier 13 and a rubber 52 extends between the movable member 40 and the inside wall of carrier 13, so that the movable member 40 is biased to stay in the neutral position in which the optical axis of the optical beam 4 is aligned with the optical axis of the objective lens 10.
The carrier 13 is constructed so that it can move over the base 53 radially to the recording carrier 1. For the purpose of focus adjustment, the movable member 40 can be driven vertically along the sliding motion shaft 51 when current is applied to the coil 313 which is fixed to the movable member 40. When current is applied to the coil 313, an electromagnetic force is developed between the coil 313 and a permanent magnet (not shown) mounted on the carrier 13 to move the movable member 40 along the shaft 51.
When current is applied to the coil 14, which serves as a linear motor together with the permanent magnets (not shown) mounted on the base 53, the carrier 13 to which the total reflection mirror 8 and objective lens 10 are mounted moves radially to the recording carrier 1 due to the electromagnetic force developed between the coil 14 and the permanent magnets.
The movable member 40 is further constructed to pivot, by a very small amount, around the sliding motion shaft 51 from the neutral position against the biasing force of rubber 52 when a current is applied to the coil 89. The coil 89 is fixed to the movable member 40 and a permanent magnet (not shown) is provided on the carrier 13. When current is applied to the coil 89, the electromagnetic force developed between the coil 89 and the permanent magnet causes movable member 40 to pivot about shaft 51 by a small degree. This results in the pivotal movement of the objective lens 10 mounted on the movable member 40 in a direction substantially perpendicular to the track extending direction in the range of several hundred micrometers (.mu.m), so as to control the tracking error (or track deviation error). Hereinafter, the assembly for moving the objective lens 10 in a direction perpendicular to the tracks, i.e., in the radial direction of the disk 1, by applying a current to coil 89 is referred to as the "tracking actuator."
The reflected light 9 from the recording carrier 1 passes the objective lens 10, is reflected by the total reflection mirror 8, passes the 1/4 wavelength plate 7, and is reflected by the polarization beam splitter 6 into the detection lens 81. Part of the reflected light 9 passing the detection lens 81 is incident on the photodetector 303.
The photodetector 303 has a two part construction, and the output from each part is input respectively to the amplifiers 304, 305. The outputs from the amplifiers 304, 305 are input to the corresponding terminals of the differential amplifier 306, which outputs the difference signal of the two inputs.
This configuration forms a focus error signal detection system commonly known as the knife edge method, and results in a focus error signal indicating the offset between the recording carrier 1 and the focusing point of the optical beam 4 constricted by the objective lens 10. The focus is controlled by applying current to the coil 313 according to this focus error signal to drive the objective lens 10 up or down so that the focal point of the optical beam 4 is positioned on the recording carrier 1.
It is to be noted that the phase compensation circuit 307 is used to maintain an acceptable phase margin in the focus control system, and the drive circuit 311 is the power amplifying circuit.
The light source 3, coupling lens 5, polarization beam splitter 6, 1/4 wavelength plate 7, detection lens 81, reflecting mirror 82, and photodetectors 11 and 303 are secured to the base 53 of the device. Part of the reflected light 9 passing the detection lens 81 is also incident on the other photodetector 11.
This photodetector 11 also has a two part construction, and the output from each part is input respectively to the amplifiers 16, 17. The outputs from the amplifiers 16, 17 are input to the corresponding terminals of the differential amplifier 18, which outputs the difference signal of the two inputs. The output of the differential amplifier 18 is used as the tracking error signal TE indicating the offset between the track and the optical beam 4 spot focused on the recording carrier 1. This configuration forms a detection system commonly called a "push-pull method."
The output signal from the differential amplifier 18 is input to the phase compensation filter 300, which corrects the phase of the tracking control system. The output of the phase compensation filter 300 is input to the power amplifying drive circuit 35 and equivalent filter 301. The drive circuit 35 supplies the current to the coil 89 according to the input signal. Thus, the movable member 40 pivots by an amount and in the direction determined by the TE signal from the neutral position against the biasing force of rubber 52. The objective lens 10 on the movable member 40 is thus shifted in the radial direction of the disk 1 by a small amount, in the order of several hundred micrometers, to correct the tracking error.
According to the push-pull method for correcting the deflection between the beam center and the track center, the objective lens 10 is pivoted by the pivotal movement of the movable member 40. However, when the objective lens 10 is pivoted by an amount determined by the tracking error signal TE, the optical axis of the objective lens 10 deviates from the optical axis of the beam entering the objective lens 10, resulting in a quasi-tracking error. To eliminate the quasi-tracking error, carrier 13 is moved in a similar manner to the pivotal movement of the movable member 40. To this end, equivalent filter 301 is provided which simulates the pivotal movement of the movable member 40 with respect to the tracking error signal TE.
The transfer function of the equivalent filter 301 is equivalent to the relationship of the displacement of the objective lens 10, mounted on the movable member 40, to the current supplied to the coil 89. In other words, the equivalent filter 301 is a simulator which gives an output equal to the amount and direction of the shift of the objective lens 10 in response to the input TE signal. The output of the equivalent filter 301, which is referred to as a simulation signal SS, expresses the displacement of the objective lens 10 for correcting the quasi-tracking error.
The transfer function of the equivalent filter 301 is determined by the amount of displacement of the objective lens 10 with respect to the input TE signal. Because rubber 52 is used to hold the movable member 40 in the neutral position, and the pivotal movement is effected against the rubber force, it is necessary to consider the characteristics of the rubber 52 in the transfer function. Since rubber 52 shown in FIG. 1 typically has both spring characteristics and viscosity, the transfer function has a quadratic form as shown in equation 1. ##EQU1## where Ke is the spring constant, Kf is the thrust constant, D is the viscosity coefficient or the damping factor, m is the mass of the movable member 40, including the objective lens 10, and S is a Laplacian factor expressed in complex number. The equivalent filter 301 is thus an electrical circuit with the transfer function defined in equation (1).
The equivalent filter 301 produces the simulation signal SS indicative of pivotal movement of the objective lens 10 about the shaft 51, and the simulation signal is applied to the coil 14 of the linear motor through the phase compensation filter 302 and the drive circuit 36 to assure the phase margin of the carrier 13 control system. Therefore, the carrier 13 moves in the radial outward direction to move the objective lens 10 together with the carrier 13 to a position to counterbalance the quasi-tracking error, resulting such that the displacement of the optical axis of the objective lens 10 with respect to the aiming track becomes substantially zero.
It should be noted that the control band of the tracking actuator is typically approximately 2 kHz, and the control band of the carrier 13 is several hundred hertz.
The problem with this conventional tracking control apparatus is that the amount of pivotal movement of the objective lens 10 varies with respect to the temperature change, because the viscosity constant of the rubber varies with temperature. However, such a variation in the amount of the pivotal movement of the objective lens due to the temperature change is not effected in the equivalent filter 301. According to the prior art tracking control apparatus of FIG. 1, the equivalent filter 301 is designed to simulate the pivotal movement of the objective lens 10 at the fixed temperature, such as 20.degree. C. Therefore, when the temperature of the rubber 52 changes from 20.degree. C., the simulation signal SS as produced from the equivalent filter 301 is not equal to the actual pivotal movement of the objective lens 10. As a result, the high precision tracking control is not possible, and the control system becomes unstable the greater the temperature difference becomes.